The retina is supplied with oxygen through both choroidal and retinal circulation. Oxygen diffusion from the choroid supplies oxygen to the avascular outer retina and photoreceptors, while the inner retina and nerve fiber layer is supplied with oxygen via the retinal vascular system (arteries, veins and capillary beds).
Like other neural tissues, the retina is very metabolically active and has a high oxygen demand. Consequently, diseases that alter retinal circulation, such as diabetic retinopathy, sickle cell disease, hypertension, and vascular occlusive diseases, result in impaired oxygen delivery. As a consequence, optical function can be significantly impaired, and severe damage to retinal tissues can occur.
The specific problem to which the invention is applicable is optical retinal oximetry--the use of noninvasive optical means to determine hemoglobin oxygen saturation in the inner retina. More generally, the invention is applicable to optical hemoglobin oximetry techniques f or use with any tissues (such as vascular beds) that are optically accessible.
In the context of the specific problem, retinal oximetry techniques have been used in the clinical evaluation of retinal diseases such as diabetic retinopathy. In diabetes, progressive retinal vascular disease leads to retinal ischemia, which in turn induces production of vasoproliferative factor that eventually results in neovascularization with its multiple complications. (References 1,2,3,4 at the end of the Background). The current front line treatment for neovascularization is laser panretinal photocoagulation (PRP). This treatment modality has proven to be extremely effective, but exactly why it is effective is unclear. (References 5,6,7,8)
Three proposed mechanisms may contribute to the regression of diabetic retinal neovascularization to greater or lesser degrees. The first of these suggests that the total retinal metabolic demand is reduced by ablating hypoxic retinal tissue with PRP. The second proposes that the PRP scars somehow alter the circuitory relationship between the choroid and hypoxic inner retina thereby allowing improved oxygen delivery. Alternatively, it has been found that laser scars cause the retinal pigment epithelium (RPE) to release a protein that directly inhibits neovascularization.
It seems likely that more than one, if not all, of these mechanisms contribute to neovascular regression post PRP treatment. What has been lacking is an accurate and available means by which to evaluate retinal metabolism, oxygenation and development of neovascularization.
Invasive techniques to provide such information are not practical for routine clinical use or research. Consequently, several techniques for noninvasive evaluation of retinal metabolism have been developed.
The first of these has looked at retinal blood flow as a measure of retinal metabolism. (References 9,10) Efforts to noninvasively evaluate retinal oxygen metabolism remained relatively stagnant however, until the principles of optical hemoglobin oximetry were first applied to the retina in 1963. (Reference 11)
Techniques for optical oximetry have been evolving, and have been applied to both animals and humans. (References 12,13) Early efforts were based on using only two wavelengths of light filtered from incandescent light sources. Unfortunately, this led to inaccurate results because of the Lambert-Beer Law, which strictly limits two wavelength oximetry to only hemolyzed solutions.
In 1975, these limitations for non-hemolyzed blood were overcome by the development of an accurate three-wavelength technique for the measurement of percent oxyhemoglobin. (Reference 14) Three-wavelength oximetry is based on several important principles. The first of these states that light absorption by blood depends on O.sub.2 Sat and wavelength. Second, a relationship exists between a measurable optical quantity like optical densities and the extinction coefficient of the mixture of oxyhemoglobin (HbO.sub.2) and deoxy-hemoglobin (Hb) at a given O.sub.2 Sat. (Reference 15) Finally, optical densities at two specific wavelengths can be compared to the optical density at a third specific wavelength and hemoglobin absorption values may then be calculated and used to accurately obtain percent O.sub.2 Sat. (Reference 14)
The relationship between the extinction coefficients of HbO.sub.2 and Hb in the visible spectrum are available for use when calculating percent O.sub.2 Sat. The advantages and disadvantages of these wavelengths using existing technology have previously been explored. (References 16,17)
In particular, wavelength selection criteria for retinal oximetry have been based on inherent characteristics of retinal tissue and blood. Wavelengths significantly below 520 nm have not been explored because of strong absorption by hemoglobin, leading to relatively low retinal tissue reflectance, which would require intense polychromatic illumination.
Three wavelength oximetry has been adapted to real-time measurements of retinal vessel oxygen saturations (O.sub.2 Sat). (References 15,16,17) These retinal oximeters use a bright source of non-collimated light (typically a broad spectrum halogen or arc light) that is filtered to provide three selected wavelengths--the light source and the filters are cooperatively selected to provide at least one isobestic wavelength (i.e., a wavelength at which hemoglobin absorption is essentially independent of 02 Sat) and at least one wavelength for which blood absorption is dependent upon O.sub.2 Sat.
To probe a selected area of the retina, the light is focused on either a large caliber retinal artery or a large caliber retinal vein. The percent O.sub.2 Sat is calculated from measurements of the light reflected from either the artery (in which hemoglobin oxygenation is relatively high) or the vein (in which hemoglobin oxygenation is relatively low), and from the RPE background.
This technique for performing retinal oximetry is disadvantageous in several respects. It is complicated to control, requiring precise focusing on retinal blood vessels and a complicated filtering to produce a multiwavelength probe. It limits percent O.sub.2 Sat measurements to large caliber blood vessels, and does not allow O.sub.2 Sat measurements to be made in the intraretinal capillary beds. It is based on measuring reflected light, and therefore uses wavelengths up in the reflectance (or partial transmittance) band above 600 nm, which tend to yield low contrast for the vascular blood column, and to provide less uniform reflections with variable background fundus pigmentation. Because, it requires a bright light source (to ensure adequate reflectance for measurement), it has a tendency to probe the sub-retinal choroidal blood, which makes it difficult to isolate the retina for O.sub.2 Sat measurement.
Optical oximetry techniques have been used to determine hemoglobin oxygenation in non-ocular tissues. For example, optical oximeters are used to measure oxygen saturation in relatively thin tissues (such as the ear lobe or the finger) through which probe light of wavelengths substantially longer than 520 nm can be transmitted. Because of the high level of optical absorption in hemoglobin even for wavelengths longer than 520 nm, the reliance on the transmission of probe light through the tissue significantly restricts the applications for these oximetry techniques.
Accordingly, a need exists f or an improved optical oximetry technique, one that has specific applicability to determining hemoglobin oxygenation in the capillary beds of the inner retina, and more general applicability to other light-accessible tissues.